JP6499311B2 - Light guide system for physiological sensors - Google Patents

Description

  Physiological sensors such as heart rate sensors based on one or more emitters and detectors have been developed for use in health and exercise monitoring. These sensors may be integrated into earphones or other wearable forms for positioning in proximity to or against the wearer's skin. Such devices often cause discomfort to the wearer during movement or exercise and can degrade performance due to ambient light.

  In one aspect, a light guide system for a physiological sensor having an emitter and a detector includes a pair of light channels. Each of the light channels is formed of a biocompatible elastic plastic and is configured to contact the wearer's skin. One of the optical channels is in optical communication with the emitter and the other optical channel is in optical communication with the detector. The biocompatible elastic plastic has a light transmission spectrum that transmits the light energy of the emitter and substantially blocks ambient light.

  Embodiments of the light guide system can include one of the following features, or any combination thereof. The light transmission spectrum may define a long pass optical filter in which the shortest wavelength of the emitter's light spectrum is greater than the transition wavelength of the long pass optical filter. The biocompatible elastic plastic can be dyed silicone. The optical channel can be different structures such as wristbands, finger bands, watches, chest straps, armbands, necklaces or ear tips for earphones.

  In another aspect, the physiological sensor includes a sensor body, an emitter, a detector, a processor, and a pair of light channels. The emitter is disposed within the sensor body and is configured to irradiate the wearer's skin with a light beam. The detector is disposed within the sensor body and is configured to receive light energy from the irradiated skin of the wearer and generate an electrical signal in response to the received light energy. The processor is configured to communicate with the detector and determine the wearer's physiological data in response to the electrical signal. Each light channel is formed of a biocompatible compatible plastic and is configured to contact the wearer's skin. One of the optical channels is in optical communication with the emitter and the other optical channel is in optical communication with the detector. The biocompatible compatible plastic has a light transmission spectrum that transmits the light energy of the emitter and substantially blocks ambient light.

  Physiological sensor embodiments may include one of the above and / or the following features, or any combination thereof. The emitter can be a light emitting diode. The physiological sensor can further include an ear tip that is attachable to the earphone and in which a pair of light channels are disposed. The sensor body can be a wristband, finger band, watch, chest strap, armband, necklace or earphone.

  In another aspect, a light guide system for a physiological sensor includes an emitter, a detector, and an ear tip. The emitter is configured to irradiate the wearer's skin with a light beam. The detector is configured to receive light energy from the irradiated skin of the wearer and generate an electrical signal in response to the received light energy. The ear tip includes an ear tip body and a pair of optical channels formed integrally with the ear chip body. Each light channel is formed of a biocompatible elastic plastic and is configured to contact the wearer's skin. One of the optical channels is in optical communication with the emitter and the other optical channel is in optical communication with the detector. The biocompatible elastic plastic has a light transmission spectrum that transmits the light energy of the emitter and substantially blocks ambient light.

  Embodiments of a light guide system for a physiological sensor can include one of the above and / or the following features, or any combination thereof. The light transmission spectrum can define a long pass optical filter in which the shortest wavelength of the light spectrum of the emitter is longer than the transition wavelength of the long pass optical filter. The biocompatible elastic plastic can be dyed silicone.

  In the following description, reference is made to the accompanying drawings. In the drawings, like reference numbers indicate like elements and features in the various figures. For clarity, not all elements are labeled in every figure. The drawings focus on explaining specific principles and are not necessarily to scale.

FIG. 5 is a graph of light transmission as a function of wavelength for a sample of translucent silicone with a thickness of 1.72 mm and a thickness of 3.35 mm. FIG. 6 is a graph of light transmission as a function of wavelength for a blend of silicone and Lumogen® Black 4280 pigment at a pigment concentration of 2%. It is a perspective view of an example of the left ear tip for the optical heart rate sensor arranged in the earphone body. It is a perspective view of an example of the left ear tip for optical heart rate sensors in an earphone body. It is a perspective view of an example of a right ear tip for an optical heart rate sensor in an earphone body. It is a perspective view of another example of the right ear tip for the optical heart rate sensor in the earphone main body. FIG. 6 is a perspective view of another example of a right ear tip for an optical heart rate sensor within an earphone body, where the position and holding structure is shown as a cutaway view. It is a perspective view of another example of the right ear tip for the optical heart rate sensor in the earphone main body. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 2 is an end view of multiple light channel configurations for a light guide system that can be used in various implementations of physiological sensors. FIG. 5 shows an example of a watch case placed on a silicone bottom layer having two light channels for use with a physiological sensor placed in the watch case. FIG. 6 shows an example of a wristband having a band body and a silicone bottom layer having two light channels for use with a physiological sensor disposed in or on the band body.

  The terminology used herein is for the purpose of describing particular examples and embodiments and is not intended to be limiting. All terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art unless otherwise defined herein.

  The term “physiological” refers to a characteristic of the function of the human or animal body. As used herein, the term “skin” generally refers to human or animal skin and includes the surface of internal and external features of the ear. By “biocompatible” material is meant any material that does not substantially adversely affect when in contact with the skin. For example, biocompatible materials are not toxic to the skin, are not abrasive, are chemically inert to the skin, are non-allergenic, and do not cause discomfort in humans or animals.

  As used herein, the term “sensor” is a signal that detects or senses energy (eg, heat, light, sound, etc.) and responds to parameters (eg, temperature, intensity, sound pressure, pressure, etc.). Refers to a device that generates For example, the optical sensor can detect light energy and generate a signal that is responsive to a parameter (eg, intensity) of light incident on the sensor. In some examples described below, the sensor includes a combination of components including an emitter for generating light energy and a detector for detecting light energy.

  As used herein, “monitor” refers to a device for monitoring or observing detected parameters. In some embodiments and examples described below, the monitor includes a sensor and generates an output, such as a display or audio signal, that can be viewed or heard based on the signal generated by the sensor.

  The term “body” typically refers to an object or structure. The body is placed on the surface of the body where one or more components are placed inside the body (eg, embedded in the body or placed in a body cavity) or using fasteners or connectors Or may include features that allow it to be attached to the body via a plurality of intervening structures.

  The term “earphone” refers to an earphone that is at least partially insertable into the ear. Earphones include canal horns and similar devices. The term “ear tip” is synonymous with the term “earphone tip” and refers to a removable and replaceable component that can be attached to the body of the earphone. Ear tips are used to seal the ear canal from ambient sounds.

  Physiological monitors for the general public have become popular in recent years. These monitors are often in the form of wearable fitness tracking devices such as wristbands or can be integrated into devices that provide additional functionality. Sensors for physiological monitors can be worn at any of a variety of locations on the body including the wrist, chest, fingers, arms and neck. Specific examples include earphones and watches that have a heart rate sensor that allows the wearer to perform the primary function of monitoring the heart rate while providing audio output or time display to the wearer.

  One type of heart rate sensor is based on optical technology in which light from one or more optical emitters such as light emitting diodes (LEDs) illuminate the wearer's skin. Some of the light is reflected from the irradiated skin, while some of the light passes under the surface of the skin and reaches into tissues that contain vascular structures such as veins, arteries, capillaries. The majority of light backscattered from beneath the skin surface is typically time invariant. However, the intensity of backscattered light from blood typically varies with time. This time variation is caused by a change in the light absorption spectrum of blood due to the oxygenation variation of each pulse. Intensity variations can also be based on changes in blood volume exposed to light during each pulse. The photodetector receives a portion of the backscattered light and generates a signal having a constant (DC level) component and a time-varying (AC level) component. The signal is first supplied to an analog front end (AFE) for preprocessing or directly to a processor that determines the wearer's heart rate from the dominant frequency of the time-varying component of the signal.

  The heart rate sensor can be worn at a body location where the time-varying component of the signal is important. A heart rate sensor typically has at least one LED emitter and one or more photodetectors. The emitter and detector are typically located within or on the sensor body. For example, the sensor body can be an earphone. The first region extending from the emitter to the surface of the sensor body and the second region extending from the detector to the surface of the sensor body are typically formed of a hard plastic such as polycarbonate and are red. Includes at least part of the outer (NIR) spectrum and the visible spectrum. Usually, the two regions are separated from each other by an opaque region that prevents direct passage of light between the two regions. In some examples, the hard plastic extends beyond the surface of the sensor body and is configured to be in direct contact with the skin. The processor is typically located remotely from the sensor body and is in electrical communication with the sensor via a cable having one or more wires.

  The heart rate sensor in the earphone can be offset from the irradiated skin of the ear, so that much of the light energy from the emitter is via attenuation in the air gap and the interface between the ear tissue and air, And lost due to reflections at the refractive index discontinuities at the hard plastic-air interface. The AC level component of the signal, ie the signal to noise ratio (SNR) of the modulated light energy incident on the detector, decreases with increasing light energy loss. Heart rate monitors and other physiological monitors based on skin illumination and detection of backscatter and reflected light can be further adversely affected during exercise or other physical activity. More specifically, the position of the emitter and detector relative to the skin can change when the wearer is moving. For example, the position of the earphone in the wearer's ear may frequently shift during travel or other forms of exercise.

  Light loss due to the transmission of light through the air limits the acceptable separation of the sensor from the skin and, in general, despite the light loss at the refractive index discontinuity remains substantially constant. As separation increases, more light output is required. Furthermore, the presence of an air gap provides a means for ambient light to reach the detector, thereby reducing the SNR. The earphone body can be increased in size to eliminate the air gap. However, contact between the rigid body and the ear can cause discomfort to the wearer, including pain or pain after prolonged use. The SNR of the detected optical signal can be improved by increasing the optical energy. Increasing the light power of the emitter can shorten the battery life of the device, and if the sensor is in a recess in the earphone body, dust will collect over time and ear wax or salt from the sweat will be recessed. May accumulate. As a result, optical signal transmission can be reduced.

  Eartips are sometimes used with earphones to seal the ear canal from the surrounding environment and stabilize the position of the earphones relative to the ears. The ear tip opening passes acoustic energy through the inner ear, while the contact seal between the ear tip and the ear canal helps to reduce or eliminate background acoustic noise heard by the wearer. Ear tips are typically formed from materials that can be worn comfortably, including soft polymers such as silicone that can fit in complex ear shapes and ear canals.

  FIG. 1 shows the light transmission versus wavelength of translucent silicone for 1.72 mm and 3.35 mm thick samples (plots 10 and 12, respectively) that can be used for ear tips. High transmission without significant variation is observed over the entire visible spectrum and over the near infrared (NIR) spectrum up to about 1150 nm. Silicone is recognized as safe for contact with human tissue and comfort for earphone users. However, a wide transmission spectrum that includes NIR wavelengths below the visible spectrum and the spectrum of the emitter causes ambient light to reach the detector, degrading the SNR of the heart rate sensor.

  In the particular example of a light guide system for a physiological sensor described herein, the system includes an optical channel in optical communication with the emitter and an optical channel in optical communication with the detector. The light channel is formed of a biocompatible elastic plastic, such as silicone, mixed with a colorant. Each optical channel has a light transmission spectrum that passes through the emitter spectral band and rejects most of the light energy at shorter wavelengths. The colorant may be a pigment provided as a suspension in a liquid. Alternatively, the colorant may be a dye provided in liquid form. In some embodiments, the colorant is a mixture of pigments, a mixture of dyes, or a mixture having at least one dye and at least one pigment.

  In fact, the light transmission spectrum defines a long pass optical filter in which the shortest wavelength of the light spectrum of the emitter is longer than the transition wavelength of the long pass optical filter. As used herein, “transition wavelength” means the shortest wavelength in a wide transmittance region where the transmittance is 50% of the maximum transmittance. Each light channel is configured to contact the skin, thereby reducing or eliminating light loss due to transmission through air.

  In another example, the emitter may be a narrow band light source such as a laser diode. As described above, the optical channel can have a light transmission spectrum that passes through the emitter spectral band and eliminates most of the light energy at shorter wavelengths. Alternatively, the optical channel may have a narrow band optical transmission spectrum that passes through a narrow laser diode spectral band and excludes most of the optical energy at wavelengths below and above the emitter spectral band.

  As a specific, non-limiting example for an optical channel with long pass optical filter characteristics, FIG. 2 is a silicone and 2% pigment concentration Lumogen® Black 4280 pigment (available from BASF Corporation of Charlotte, NC) ) Shows the light transmission characteristics of the mixed material. The mixed material can be used as a suitable material for each optical channel. Plots 20, 22, 24, and 26 represent mixed silicone samples with thicknesses of 0.45 mm, 0.85 mm, 1.66 mm, and 3.31 mm, respectively. For all thicknesses, the corresponding transition wavelength is less than the shortest wavelength in the spectral band of a typical emitter for an optically based heart rate monitor. For example, the LED emitter may have a center wavelength of about 900 nm and a full width half maximum (FWHM) spectral bandwidth of about 40 nm. In this example, a light transmission spectrum with a transition wavelength close to 880 nm but less than 880 nm is preferred for maximum exclusion of ambient light.

  In one technique, the colorant is provided as a liquid mixed in gel form with liquid silicone rubber (LSR). The mixture is poured into a mold to form a resilient silicone body having the desired shape. The resulting optical channel can be formed into a larger integral body such as an ear tip for an earphone. In another embodiment of a light guide system for physiological sensors, other biocompatible elastic plastics may be used as long as the light transmission spectrum of the plastic material passes through the light energy of the emitter and substantially blocks ambient light. Used for optical channels. Accordingly, various pigments and dyes can be mixed with various plastic materials to achieve the desired light transmission spectrum.

  In various embodiments, the physiological sensor is a heart rate sensor disposed within or on the earphone. More specifically, the main body of the earphone functions as a sensor main body, and the sensor includes an emitter and a detector.

  In various examples, the optical channel is formed as a part of an ear tip that can be attached to and removed from the earphone body. When the eartip is attached, one of the light channels is located on the emitter and the other light channel is located on the detector. The cross-sectional area of each optical channel can be selected depending on the dimensions of the active region of the emitter and detector. In some examples described below, two or more optical channels are used to propagate light from one or more emitters and / or 1 to provide light to one or more detectors. More than one optical channel is used.

  FIG. 3 is a side view of the left ear tip 50 for the optical heart rate sensor arranged in the earphone main body. The ear tip body 33 is formed of an elastic plastic such as silicone modified to be opaque in the visible and near infrared spectrum. A positioning and holding structure 34 extending from the ear tip body 33 forms a means for maintaining the earphone attached to the ear tip body 33 in a stationary position relative to the ear canal. The positioning and holding structure 34 may be formed of a hard plastic, or may be formed of the same elastic plastic as the ear tip body 33, and may be transparent or opaque. The opening 36 penetrating the ear tip body 33 forms a means for the acoustic signal generated by the attached earphone to propagate to the ear canal. A pair of optical channels 38 and 40 are disposed on the top of the ear tip 30. The optical channels 38 and 40 are formed of an elastic plastic having a light transmission spectrum, and light energy from the emitter passes through the ear tip 30 and light energy back-scattered from the irradiated region of the ear is the ear tip 50. To reach the detector. When the body 33 of the ear chip 50 is attached to the earphone, the rear end (not shown) of one light channel 38 is configured to contact a protective transparent cover disposed on the emitter. Similarly, the rear end of the other optical channel 40 is configured to contact a protective cover disposed on the detector. As shown, the portion 46 of the ear tip body 33 is located between the two optical channels 38 and 40. This opaque portion 46 prevents light from the emitter from having a direct optical path to the detector.

  4 and 5 are perspective views of examples of the left ear tip 60 and the right ear tip 70 for the optical heart rate sensor in the earphone main body. The ear tips 60 and 70 have the same shape as the left ear tip 50 shown in FIG. 1, but the optical channels 38 and 40 are at different positions on the ear tip bodies 33 and 32, respectively. In particular, FIG. 4 shows light channels 38 and 40 located opposite the acoustic aperture 36 relative to the positioning and retaining structure 34. FIG. 5 shows optical channels 38 and 40 that are vertically separated and located between the acoustic aperture 36 and the positioning and holding structure 34. In both illustrated examples, the exposed ends of the two light channels 38 and 40 are configured for contact with structures near the human ear.

  FIG. 6 is a perspective view of another example of the right ear tip 30. The two optical channels 38 and 40 are located at the bottom of the ear tip body 32 and are separated by a portion 46 of the ear tip body 32.

  FIG. 7 shows another example of a right ear tip 80 in which the positioning and holding structure 34 is shown as a cutaway view to illustrate how the light channels 38 and 40 are positioned within the positioning and holding structure 34. FIG. An emitter and detector (not shown) are disposed substantially adjacent one end 42 or 44 of each of the optical channels 38 and 40, respectively. The light channels 38 and 40 are positioned such that light from the emitter illuminates a portion of the ear ring.

  FIG. 8 is a perspective view of another example of the right ear tip 90. In this example, the positioning and holding structure 48 is a separate element attached to the earphone main body 52. The removable form of the positioning and holding structure can be used in any of the examples described above. The third element in the figure is an umbrella-type eartip 54 attached to the earphone body 52, which can be used in any of the examples described above. In this example, the light channels 38 and 40 are disposed within the earphone body 52 and may extend to or beyond the body surface. For manufacturing, the optical channels 38 and 40 can be formed as individual plugs, each plug being inserted into an opening in the earphone body 52. In an alternative example, the two optical channels 56 and 58 are coupled together at one end within a single element by a bridge portion 62. Preferably, the bridge portion 62 is selectively formed with a pigment or dye that becomes opaque to the light spectrum of the emitter, thereby eliminating the direct optical path from the emitter to the detector.

  9A-9F show end views of multiple optical channel configurations that can be used in accordance with various embodiments.

  FIG. 9A shows a single optical channel 64 that may be used when crosstalk between the emitter and detector is not critical. Preferably, the separation between the emitter and the detector is increased compared to other embodiments to reduce crosstalk. On an extension of this variation, the entire ear tip is made of long pass silicone to prepare for manufacture. In another variation, the central portion of a single light channel is made opaque, so that the single portion can function as two independent light channels. Alternatively, if the entire ear tip is made of long pass dyed silicone, the area separating the emitter and detector optical paths can be made opaque to reduce or eliminate crosstalk.

  FIG. 9B illustrates the separation of the two optical channels 38 and 40 as described above for the various embodiments.

  9C, 9D and 9E are adapted for optical communication with an emitter having a plurality of optical channels 68 surrounding the central optical channel 66, and are adapted for optical communication with one or more detectors. Indicates.

  FIG. 9F shows a group 72 of optical channels 66 configured to be in optical communication with a plurality of emitters and a second group 74 of optical channels 68 configured to be in optical communication with a plurality of detectors. . The illustrated group of optical channels 66 may be advantageous for implementation on ear tips having complex shapes. In some embodiments, materials with fewer dyes and pigments are used. In other embodiments, grouping allows multiple measurements to be averaged or acquired and averaged.

  The various examples described above relate to ear tips for earphones. FIG. 10 shows an example of a watch case 80 in which a watch case body 82 is disposed on the silicone bottom layer 84. Two optical channels 86 and 88 having similar transmission characteristics as the optical channel of the example described above include a layer 84 and are used in combination with an optical physiological sensor such as a heart rate monitor disposed within the watch case body 82. Is done.

  FIG. 11 shows an example of a wristband 90 (eg, a watchband) that includes a band body 92 with a silicone bottom layer 94. Two optical channels 86 and 88 similar to those shown in the watch case 80 of FIG. As shown in FIG. 10, it allows the passage of optical energy emitted from an optical physiological sensor located in or on the band body 92.

  In general, light guide systems for physiological sensors according to the principles described herein allow for improved performance and provide wearer comfort. Several embodiments and examples have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the principles described herein. For example, the size, location, and number of optical channels may differ from those described above in various implementations.

30 Ear chip 32 Ear chip body 33 Ear chip body 34 Holding structure 36 Acoustic aperture 38 Optical channel 40 Optical channel 42 End 44 End 46 Opaque portion 48 Holding structure 50 Left ear chip 52 Earphone main body 54 Ear chip 56 Optical channel 58 Light Channel 60 Left ear tip 62 Bridge portion 64 Optical channel 66 Optical channel 68 Optical channel 70 Right ear tip 72 Group 74 Second group 80 Watch case 82 Watch case body 84 Silicone bottom layer 86 Optical channel 88 Optical channel 90 Wrist band 92 Band body 94 Silicone bottom layer

Claims (24)

A light guide system for a physiological sensor having an emitter and a detector, comprising a pair of light channels, each of the light channels being formed of a biocompatible elastic plastic and the wearer's skin is configured to contact the one of the optical channels is in communication with the emitter and optically, other optical channels has in optical communication with the detector, wherein the biocompatible resilient plastics, the emitter A light guide system having a light transmission spectrum that transmits light energy and substantially blocks ambient light.   The light guide system of claim 1, wherein the light transmission spectrum defines a long pass optical filter, and the shortest wavelength of the light spectrum of the emitter is greater than a transition wavelength of the long pass optical filter.   The light guide system according to claim 2, wherein a transition wavelength of the long-pass optical filter is less than 880 nm.   The light guide system according to claim 1, wherein the biocompatible elastic plastic is dyed silicone.   The light guide system according to claim 1, wherein the emitter and the detector are disposed in an earphone.   The light guide system according to claim 1, wherein the optical channel is disposed on an ear tip for an earphone.   The light guide system according to claim 6, wherein the optical channel and the ear tip are formed as an integrally molded body.   7. The light guide system of claim 6, wherein the light channel and the eartip are formed of the same biocompatible elastic plastic.   9. The light guide system of claim 8, wherein the light channel comprises dyed silicone and the eartip comprises the same silicone that is not dyed.   The light guide system according to claim 6, wherein the light channel is formed as a plug disposed in an opening of the ear tip.   The light guide system of claim 1, further comprising a bridge portion extending between one end of each of the light channels and formed from the biocompatible elastic plastic.   The optical channel is embedded in the earphone body, each of the optical channels extends from one of the emitter and the detector, and the bridge portion is disposed along a surface of the earphone body. The light guide system according to claim 11.   The light guide system of claim 1, further comprising at least one additional light channel and at least one additional emitter.   The light guide system of claim 1, wherein the light channel is disposed in one of a wristwatch, a wristband, a finger band, a chest strap, an armband, and a necklace. A sensor body;
An emitter disposed in the sensor body and configured to irradiate the wearer's skin with a light beam;
A detector disposed on the sensor body and configured to receive light energy from the irradiated skin of the wearer and to generate an electrical signal in response to the received light energy;
A processor configured to determine physiological data of the wearer in response to the electrical signal;
A pair of optical channels formed of biocompatible plastic and each configured to contact the wearer's skin, wherein one of the optical channels is in optical communication with the emitter; of which the optical channels in optical communication with the said detector, the biocompatible plastic may transmit light energy emitter has a light transmission spectrum for blocking ambient light substantially a pair of optical channel When,
A physiological sensor comprising:   The physiological sensor according to claim 15, wherein the sensor body is an earphone.   The physiological sensor according to claim 15, wherein the emitter is a light emitting diode (LED).   The physiological sensor according to claim 16, further comprising an ear tip attachable to the earphone, wherein the pair of optical channels are disposed on the ear tip.   The physiological sensor according to claim 15, wherein the sensor body is a part of one of a wristwatch, a wristband, a finger band, a chest strap, an arm band, and a necklace.   The physiological sensor of claim 15, wherein the physiological data is heart rate data. A light guide system for a physiological sensor,
An emitter configured to irradiate the wearer's skin with a light beam;
A detector configured to receive light energy from the irradiated skin of the wearer and generate an electrical signal in response to the received light energy;
An ear tip having an ear tip body and a pair of optical channels integrally formed in an ear tip bowl, each of the optical channels being formed of a biocompatible plastic and in contact with the wearer's skin is configured, one of said light channel, said has emitter and in optical communication with, the other of the optical channel, the detector and the optically are communicated, the biocompatible plastic, wherein An ear tip having a light transmission spectrum that transmits light energy of the emitter and substantially blocks ambient light ;
A light guide system, characterized in that it comprises a. The light guide system of claim 21, wherein the light transmission spectrum defines a long pass optical filter, and the shortest wavelength of the optical spectrum of the emitter is greater than the transition wavelength of the long pass optical filter.   The light guide system of claim 21, wherein the biocompatible plastic is dyed silicone.   The light guide system of claim 21, wherein the electrical signal indicates a wearer's heart rate.